Nowadays, a storage-virtualizing technique is known such that at least one storage is virtualized to provide logical storage devices (LUN: Logical Unit Number) for respective host devices and to enable flexible system operation without physical limitation by the storage.
FIG. 11 is a schematic diagram illustrating a typical configuration of a conventional virtualized storage system. In this embodiment, the virtualized storage system 600 includes two hosts 501a and 501b, and a RAID (Redundant Arrays of Inexpensive Disks) device 605.
The RAID device 605 includes a queue 606, a controller 603, and a disk array 602. The RAID device 605 is connected to the hosts 501a and 501b through ports 601a and 601b, respectively, and writes or reads out data for HDDs 604-1 to 604-4 of the disk array 602 according to an I/O command received from the hosts 501a and 501b. 
The disk array 602 includes the HDDs (four devices in FIG. 11) 604-1 to 604-4, and can treat the HDDs 604-1 to 604-4 as one logical disk in the RAID device 605.
The queue 606 is a storage area (memory) for storing various pieces of information for execution of the I/O command transferred to the RAID device 600 from the hosts 501a and 501b. For example, the queue 606 stores a return value for execution of the I/O command, various pieces of information representing progress on a process, a status, and the I/O command itself.
The queue 606 is divided into multiple queue resources having a predetermined size (for example, 256 bytes) in order to store the information for execution of the I/O command.
The controller 603 provides logical devices (LUNs) for the respective hosts 501a and 501b, and controls writing and reading out for the HDDs 604-1 to 604-4 according to the I/O command received from the hosts 501a and 501b. 
The controller 603 receives the I/O commands from the hosts 501a and 501b, and then assigns queue resources of the queue 606 to the I/O commands to control the HDDs 604-1 to 604-4.
Accordingly, when the hosts 501a and 501b specify addresses for the respective LUNs provided from the RAID device 605 to send the I/O commands, the controller 603 receives the I/O commands to store (assign) the I/O commands in the queue 606 in sequence.
Then, the controller 603 writes or reads out data for the HDDs 604-1 to 604-4 according to the I/O commands to which the queue resources are assigned.
When the various pieces of information for execution of the I/O command received from the hosts 501a and 501b cannot be stored due to, for example, lack of free space of the queue 606, in other words, when the queue resources cannot be assigned; the I/O command is rejected, and is returned together with a status (QFULL) representing full space of the queue 606 toward the originating hosts 501a and 501b (QFULL response).
In the virtualized storage system 600, preferably, the I/O commands are processed among the LUNs in a proper balance, and QFULL responses are minimized.
In the hosts 501a and 501b, in order to prevent application from eternally waiting for a response from the RAID device 605, a time-out period is set for the I/O command to be sent. Furthermore, when an I/O command is not processed after this time-out period (time-out), the virtualized storage system 600 is directed to cancel a process for the I/O command, and the same I/O command is retried (time-out event).
For example, Japanese Unexamined Patent Application Publication No. 2005-149436 discloses a technique for adjusting the logical size of a queue in order to minimize the occurrence of the time-out event and the QFULL event described above.
FIGS. 12(a) to (c) illustrate a technique for adjusting the logical size of the queue in a conventional virtualized storage system. FIG. 12(a) illustrates the logical size of the queue to be assigned to each queue before the adjustment. FIG. 12(b) illustrates that the LUN having a high rate of occurrence of a QFULL has a logical size increased in FIG. 12(a). FIG. 12(c) illustrates that the I/O commands for other LUNs are further received in FIG. 12(b).
In these drawings, symbols a1 to a7 denote identification information for specifying the respective LUNs. For example, in the state illustrated in FIG. 12(a), the host 501a sends I/O commands to the LUNs# a1, a2, a3, and a4 in the RAID device 605, and the queues having logical sizes are assigned to the respective I/O commands.
The technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-149436 increases the logical sizes of the queues assigned to the LUNs# a1 and a3 as illustrated in FIG. 12(b) in the case of, for example, an increase in the QFULL rates of the LUNs# a1 and a3 in the state as shown in FIG. 12(a).
This patent document also discloses a technique for decreasing the logical sizes of the queues in the case of time-out rates exceeding a threshold value in addition to the technique for increasing the logical sizes of the queues in the case of excess of the QFULL rates as described above.
Since queue resources are finite in a virtualized storage system, preferably, these queue resources are efficiently used.
However, since the LUN of the queue resources exceeding the threshold value cannot be unlimitedly increased in the conventional virtualized storage system as described above, occurrence of the QFULL events cannot be reduced.
For example, as illustrated in FIG. 12(c), the I/O commands for the LUNs# a5, a6, and a7 are further received from the host 501a in the state illustrated in FIG. 12(b) to fill the queues. After this, even if the QFULL rate of the LUN# a2 is increased, queues assigned to the LUN# a2 cannot be increased any more.
Time-out also occurs due to temporary hardware abnormality (for example, a medium error or a malfunction of a disk) of the HDDs 604-1 to 604-4 in the RAID device 605. Since occurrence of time-out due to such a temporary cause decreases the logical sizes of the queues in the conventional technique for decreasing the logical sizes of the queues in the case of the time-out rates exceeding the threshold value, the assigned queue resources are still decreased not to enable the efficient use of the queue resources even after, for example, a recovery function of the RAID device 605 fixes the hardware abnormality.